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          <h1 class="post-title" itemprop="name headline">如何区别NAS.SAN与DAS</h1>
        

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        <p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;SAN (Storage Attached Network)，即存储区域网络。为什么写NAS就不得不提到SAN呢?原因之一是它们的名字有69关系，容易混淆;之二是NAS和SAN既竞争又合作，很多高端NAS的后端存储就是SAN。NAS和SAN的整合也是存储设备的发展趋势，比如EMC的新产品VNX系列。右图展示了一台NAS的逻辑结构：双虚线框表示一台NAS。它通过Fibre Channel从后端SAN获得存储空间，创建文件系统后，再通过以太网共享给服务器。SAN提供的存储单位是LUN，属于block级别的。经过NAS创建成文件系统后，就变成文件级别的了。</p>
<p><img src="https://github.com/hcldirgit/image/blob/master/%E5%A6%82%E4%BD%95%E5%8C%BA%E5%88%ABNAS.SAN%E4%B8%8EDAS/01.jpeg?raw=true" alt=""></p>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;如果上边的逻辑图还不够清楚，可以看看下面的物理连接。NAS通过FC Switch连到SAN上，应用服务器再通过Ethernet Switch连到NAS上。同时SAN也直接提供block级别的存储给应用服务器。</p>
<p><img src="https://github.com/hcldirgit/image/blob/master/%E5%A6%82%E4%BD%95%E5%8C%BA%E5%88%ABNAS.SAN%E4%B8%8EDAS/02.jpeg?raw=true" alt=""></p>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;关于NAS和SAN的区别，可以列出很多来。比如带宽大小，距离长短，共享优劣等等。几乎所有区别都是由两个因素衍生出来的。一个是FC与Ethernet，另一个是block与file system。简而言之，如果用户需要通过FC访问block，就用SAN;如果需要通过Ethernet访问file system，就用NAS。</p>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;除了NAS和SAN，还有一类存储设备经常被提到。那就是DAS (Direct Attached Storage) ，即“直连存储”。“直连”指服务器和存储设备之间没有FC网络，而是直接相连。比如我们都熟知的个人电脑就是DAS，因为磁盘被直连到了主板上。DAS已经存在很多年了，就算到今天也是很多服务器的理想选择。但是它的问题很多，而且也跟不上IT技术，比如虚拟化的发展。下面列举几个：</p>
<ol>
<li><p>可管理性差：每台服务器都使用自己的存储，光硬件的监控和维护就要花费不少时间。如果都要做容灾或备份，对于管理员简直是噩梦。</p>
</li>
<li><p>可扩展性差：在服务器安装结束后，如果发现存储空间分配过多，就造成了浪费。如果发现空间不足，要扩展也很麻烦。</p>
</li>
<li><p>跟不上IT发展趋势，比如不支持VMware VMotion等高级功能。</p>
</li>
</ol>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;SAN解决了这些问题。因为它既提供了统一的存储，同时又是一个网络。统一性和网络性给SAN带来了很多优势：</p>
<ol>
<li><p>可管理性：由一台SAN统一给多台服务器提供存储。无论是硬件的监控维护，还是数据的容灾备份，都只要在SAN上进行。使存储管理变得更轻松。</p>
</li>
<li><p>可扩展性：在物理层面，SAN支持数以百计的磁盘(比如EMC的CX4可以支持960块磁盘)，提供了海量的存储空间。在逻辑层面，这个海量空间可以按需要分成不同大小的LUN，再分配给服务器。LUN是逻辑设备，所以很容易扩展或迁移。</p>
</li>
<li><p>符合IT发展趋势：比如对炙手可热的虚拟化有很好的支持。</p>
</li>
</ol>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;当然，除了解决DAS的这些问题，SAN还有其他明显的优势：</p>
<ol>
<li>高性能:<br>a. SAN 更好的支持RAID，因为它拥有更多硬盘和更强的控制器。下图展示了RAID0对性能提升的基本原理：当有一大块数据写到RAID Group上，它可以被分成数小块，同时写到几个磁盘上。这就象有一批档案需要录入到电脑上，经理一个人打字需要做5天。分给5位员工一起做，一天就可以做完了。</li>
</ol>
<p><img src="https://github.com/hcldirgit/image/blob/master/%E5%A6%82%E4%BD%95%E5%8C%BA%E5%88%ABNAS.SAN%E4%B8%8EDAS/03.jpeg?raw=true" alt=""></p>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;b. SAN有更大的cache。比如CX4的write cache可以达到10.7 GB。Cache对性能的提高也有明显的作用。</p>
<ol>
<li><p>更稳定：多机头，热备盘，多路径等机制杜绝了单点故障。</p>
</li>
<li><p>更安全：统一的容灾，备份和远程复制保证了数据的安全性。</p>
</li>
</ol>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;此外还有很多新技术，比如VNX的FASTCache和FASTVP。因为主要介绍NAS，SAN就不深入讨论了。</p>
<p><img src="https://github.com/hcldirgit/image/blob/master/%E5%A6%82%E4%BD%95%E5%8C%BA%E5%88%ABNAS.SAN%E4%B8%8EDAS/04.jpeg?raw=true" alt=""></p>

      
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